The efficient use of water source is required due to the industrial development and population increase. In order to realize the efficient use of water source, it is very important to reuse waste water which requires the purification of water and separation of other substances from water.
On the other hand, boron is widely employed in various technical fields such as semiconductor manufacture, control rod in nuclear power plant and glass manufacture depending on the specific characteristics thereof, and thus becomes a requisite element in high-tech industry. However, boron is harmful for human and causes neurotoxicity and growth inhibition so that the emission regulation of boron is severe.
The World Health Organization (WHO) reconsidered the toxicity evaluation of boron in 1998 and lowered the water quality standard of water supply down to 0.5 ppm. Also, the Water Quality Pollution Control Act was constituted in Japan in 2001 and thus the emission standard was set to 10 ppm or less. Moreover, the purified water to be used in semiconductor manufacturing plant contains an extremely small amount of boron so that the removing technique relating to boron is required.
Boron exists as borate ion in water. As the removing method of boron can be exemplified membrance separation, electrical separation, ion-exchange and coagulation/sedimentation. Among them, particularly, the ion-exchange is widely employed due to the low running cost and low occurrence of sludge. In the ion-exchange method, ion-exchange resin with glucamine chelate as a boron adsorbent is proposed.
In this case, the recovering of the boron adsorbent after the boron adsorbent process is conducted by subsequently contacting the boron adsorbent with an acid and an alkali solution. Concretely, a first recovering liquid containing an agent such as 0.5 N to 1 N-sulfuric acid and a second recovering liquid containing an agent such as sodium hydroxide are prepared. First of all, the boron adsorbent containing the adsorbed boron is treated with the first recovering liquid to release coordinating negative ions from the boron adsorbent and then treated with the second recovering liquid to generate hydroxyl groups on the surface thereof.
According to the aforementioned method, however, in the recovering of the boron adsorbent, since it is required the first recovering liquid and the second recovering liquid are prepared and the boron adsorbent is subsequently treated with the first recovering liquid and the second recovering liquid, two recovering steps are required to complicate the recovering treatment of the boron adsorbent. Moreover, since the first recovering liquid contains the sulfuric acid or the like and the second recovering liquid contains the sodium hydroxide or the like, the thus treated liquid contains sulfate ion and sodium ion in respective high contents in addition to the released boron (i.e., negative ionic compound containing boron). It is therefore very difficult that only the boron is separated, removed and collected from the treated liquid.